1. Field of the Invention
The present invention concerns a torsional damper comprising at least two coaxial parts mounted to rotate relative to one another within a defined range of relative angular movement and circumferentially acting elastic means between said parts resisting such relative angular movement over at least part of said range of relative angular movement.
2. Description of the Prior Art
This type of torsional damper is normally incorporated in the design of a friction clutch plate, particularly for automotive vehicles, in which case one rotating part comprises a friction disk designed to rotate with a first shaft, in practice a driving shaft and the motor output shaft in the case of an automotive vehicle, whereas another of said rotating parts is carried on a hub designed to rotate with a second shaft, in practice a driven shaft and the gearbox input shaft in the case of an automotive vehicle.
This type of device is used to permit regulated transmission of rotational torque applied to one of its rotating parts where the other is itself subject to a rotational torque. It is thereby able to filter vibrations which may arise at any point in the kinematic system in which it is incorporated, extending from the motor to the driven road wheels in the case of an automotive vehicle.
In torsional dampers of this type the circumferentially acting elastic means are generally coil springs, extending in a generally tangential direction relative to a circumference of the damper assembly, each located partly in an opening formed for this purpose in a first component forming part of one of the two rotating parts (a flange, for example) and partly in an opening formed for this purpose in one or more other components forming part of the other rotating part (annular guide members, for example).
One problem to be overcome in designing torsional dampers of this type is the need to provide room for these springs in a space which is limited in the radial direction by other design factors and which is already congested, while the number of springs may be large and the springs inevitably weaken the components in which the openings to accommodate them are formed.
It is already known in the art, however, to substitute elastically deformable arms for coil springs in constituting the circumferentially acting elastic means.
Such elastically deformable arms are disclosed, for example, in French Pat. No. 2 315 042, filed June 18, 1975 under Application No. 75 190 48.
However, these elastically deformable arms form part of a special type of hydrokinetic transmission coupling incorporating a clutch mechanism, in which their specific function is to control the operation of a valve. In the transmission system disclosed, the elastically deformable arms are operative for only one direction of rotation of the rotating parts between which they act.
Furthermore, while they are attached at one end to a support connected to one of the rotating parts so as to rotate therewith, they are also positively attached to the other rotating part at their other end. Thus the assembly is designed as a specific unit, requiring special fitting.
This has no connection with the object of the present invention, which is furthermore concerned with a torsional damper, and in particular with a friction clutch plate and not with a hydrokinetic transmission coupling incorporating a clutch facility.
U.S. Pat. No. 2,114,247 is concerned with a torsional damper of the kind to which the present invention relates, and also discloses the use of elastically deformable arms as circumferentially acting elastic means.
In this American patent, each of the elastically deformable arms used is formed by winding into a helix a rod which is substantially square in transverse cross-section. This is a relatively complex operation, in that winding to the required diameter is difficult and expensive. The bulk in the axial direction is also considerable. Moreover, these arms extend continuously from one of the rotating parts concerned to the other, so that they are operative between the rotating parts as soon as relative angular movement occurs between them, and thus for low values of the torque transmitted from one rotating part to the other as well as for higher values of this torque.
Thus the circumferentially acting elastic means between said rotating parts have the same stiffness throughout relative angular movement between them.
In practice, it is beneficial for the circumferentially acting elastic means to be of relatively low stiffness at low values of torque, in particular for good absorbtion of dead point noise with the motor stationary. They should also have relatively high stiffness at higher values of torque, and this is not possible with the arrangement disclosed in U.S. Pat. No. 2,114,247.
By virtue of the relatively large bulk in the axial direction of each of the two elastically deformable arms used, the arms cannot be easily staggered in the axial direction without increasing the axial dimension of the assembly significantly, and in practice unacceptably.
Furthermore, were they to be axially staggered in this way a significant radial component of elasticity would result.
In practice, they are disposed axially in the same transverse plane, and thus can only extend circumferentially in the same direction.
This direction is naturally selected so as to correspond to operation with the torque transmitted from one of the rotating parts to the other increasing, for which the elastically deformable arms tend to roll up.
These elastically deformable arms are not well suited to operation with the aforementioned torque decreasing, however, as this tends to cause them to unroll.
The general object of the present invention is to provide a device to overcome the aforementioned disadvantages.